Fiat 124 (1966) was transmogrified into Soviet Zhiguli/Lada cars, production of which design ended in 2012 in Russia, but continued well into 2014 in Egypt. With 2014 in mind, that's 48 years of mostly continuous production, though in that same year of 2014, the plant in Egypt had a fire

With trams, it's the PCC Streetcar (1936), numerous modifications of which are still in service. I don't know, whether any new PCC-spec trams are still built.

The Boeing B-52 Stratofortress had its first flight in 1952, and these planes are still in service.